The Odiel river drains the central part of the Iberian Pyrite Belt, one of the oldest and most important sulphide mining districts of the world. As a consequence of a long mining history, large amounts of sulphides and sulphide-rich residues have been exposed in the surface. The oxidation of these sulphides has originated acidic effluents very rich in sulphates, metals and metalloids that end in the Odiel river. As a result, the water in the middle and lower part of the river normally has a pH lower than 3.5 and very high amounts of metals and sulphates. In order to understand the transport and fate of pollutants in this river, we have started a study of the river chemistry combining a systematic sampling with the analysis of water flux and metal concentration data for the last 7 5 years, from which the first results are shown here